In WCDMA many users share the same frequency band. The transmission of one user thereby becomes interference for all other users in the uplink (and downlink). Interference is a problem for any receiver as it makes it difficult for the receiver to receive the sent out signal or transmission correctly. Often, errors are introduced in the received signal as a result of interference.
In order to reduce to effects of interference, Interference Suppression, IS, receivers are used. Such IS receivers enables better performance in terms of e.g. peak data rates, coverage, system throughput and system capacity.
A scheduler in a radio base station schedules users, or user equipments, on the mobile broadband uplink, denoted EUL, trying to exploit the available load headroom in the best possible way. Since the headroom varies due to the load caused by already scheduled users, this scheduling need to be fast. The scheduling is affected by delays of several scheduling intervals though, from the time a grant is scheduled and transmitted to a terminal, until the load appears on the uplink air interface as an interference power. This means that in order to be able to schedule so that the air interface load is kept below the maximum load (needed to maintain stability and coverage of the cell), the scheduler needs to predict the load that appears over the air interface, and it needs to do that accurately. In case without interference suppressing receivers this prediction can e.g. be performed deterministically and analytically using measured SIR values, together with beta factors that define the offset of the data power of a user, to the power controlled control channel. This load prediction is associated with errors, but is deemed sufficient for non-IS receivers.
When IS receivers are employed, the load prediction problems unfortunately become severe. In particular, the load appearing over the air interface is normally significantly reduced with IS receivers, as compared to non-IS receivers, the load reduction obtained by the IS receivers vary with the number of interfering users in the cell and the load reduction obtained by the IS receivers vary also with the bit rates of the interfering users in the cell.
Predicting the load appearing over the air interface is difficult for several reasons. There is a relatively long delay between sending an uplink transmission grant to a user equipment until the actual load appears on the air interface. The delays also vary making it even more difficult to take the delay into account when trying to predict the load appearing on the air interface. Still a problem is that user equipments may use or transmit with lower power than granted, which will also affect the load on the air interface and the interference situation on the air interface.